1. Field of the Invention
The present invention relates to a running power transmission mechanism for a vehicle, which transmits drive power from a driving source to steering and steered wheels, which are respectively disposed closer to first and second sides of the vehicle in a fore and aft direction thereof.
2. Background Art
In respect to the vehicle with the steering and steered wheels respectively disposed closer to the first and second sides of the vehicle in the fore and aft direction thereof, it is known that, when turning the vehicle, the steering wheels are rotated at a higher speed than the steered wheels so as to shorten the turning radius of the vehicle and prevent such as slippage of the steering wheels. This arrangement poses problems as mentioned below.
That is, there has been proposed a transmission mechanism, in which a steering-wheel driving mechanism in the form of a geared transmission unit is installed in a steering-wheel driving train so as to allow the speed of the geared transmission unit to be changed in accordance with the operation of an operation lever. In this transmission mechanism with the geared transmission unit, the speed at which the steering wheels are driven (steering-wheel drive speed) is stepwisely changed, and therefore there causes a speed differential in changing the steering-wheel drive speed, resulting in deteriorated ride quality and likelihood of damaging a running surface such as lawn.
As another form of the steering-wheel driving mechanism, there has been also proposed an HST, which has a hydraulic pump and a hydraulic motor in the steering-wheel driving train so as to be capable of non-stepwisely changing the speed of the output of the hydraulic motor by the control of the slanting angle of a swash plate of the hydraulic pump according to the steering angle of the steering wheels.
Although the transmission mechanism with the HST can non-stepwisely change the steering-wheel drive speed and therefore does not cause the problems as mentioned above, it cause a different problem due to a specific construction of the transmission mechanism with the HST, which requires the entire power for driving the steering wheels to be transmitted through the HST to the steering wheels, and hence only the HST to change the steering-wheel driving speed. As a result, the HST must have a bigger size for achieving this function, which may invite high production cost and increased installation space.
The present invention has been conceived to solve the above problems. It is an object of the present invention to provide a running power transmission mechanism that is capable of stepwisely changing the speed of drive power transmitted to the steering wheels, as well as achieving cost reduction and decreased installation space.